On properties of families of sets Lecture 1 Lajos Soukup Alfréd Rényi Institute of Mathematics Hungarian Academy of Sciences http://www.renyi.hu/∼soukup 7th Young Set Theory Workshop “On properties of families of sets” “On properties of families of sets” • Miller, Edwin W. On a property of families of sets. C. R. Soc. Sci. Varsovie 30, 31-38 (1937). “On properties of families of sets” • Miller, Edwin W. On a property of families of sets. C. R. Soc. Sci. Varsovie 30, 31-38 (1937). • Erdos,˝ Paul; Hajnal, András On a property of families of sets. Acta Math. Acad. Sci. Hung. 12, 87-123 (1961). Outline • Property B and its Friends-and-Relations: Outline • Property B and its Friends-and-Relations: • chromatic number, transversal property, property B(κ), essential disjoint property, conflict free chromatic number, weak Freese nation property, Outline • Property B and its Friends-and-Relations: • chromatic number, transversal property, property B(κ), essential disjoint property, conflict free chromatic number, weak Freese nation property, • Elementary submodels Outline • Property B and its Friends-and-Relations: • chromatic number, transversal property, property B(κ), essential disjoint property, conflict free chromatic number, weak Freese nation property, • Elementary submodels • Shelah’s Singular Cardinal Compactness Theorem Outline • Property B and its Friends-and-Relations: • chromatic number, transversal property, property B(κ), essential disjoint property, conflict free chromatic number, weak Freese nation property, • Elementary submodels • Shelah’s Singular Cardinal Compactness Theorem • Forcing construction Outline • Property B and its Friends-and-Relations: • chromatic number, transversal property, property B(κ), essential disjoint property, conflict free chromatic number, weak Freese nation property, • Elementary submodels • Shelah’s Singular Cardinal Compactness Theorem • Forcing construction • Combinatorial principles Hilbert’s first problem Hilbert’s first problem Hilbert, 1900: The investigations of Cantor ... suggest a very plausible theorem: Every system of infinitely many real numbers . is either equivalent to the assemblage of natural integers, . or to the assemblage of all real numbers and therefore to the continuum . Hilbert’s first problem Hilbert, 1900: The investigations of Cantor ... suggest a very plausible theorem: Every system of infinitely many real numbers . is either equivalent to the assemblage of natural integers, . or to the assemblage of all real numbers and therefore to the continuum . Cantor, 1885: Every perfect subset of the reals has size continuum. Hilbert’s first problem Hilbert, 1900: The investigations of Cantor ... suggest a very plausible theorem: Every system of infinitely many real numbers . is either equivalent to the assemblage of natural integers, . or to the assemblage of all real numbers and therefore to the continuum . Cantor, 1885: Every perfect subset of the reals has size continuum. Prove: every uncountable subset of the reals contains a perfect subset. Hilbert’s first problem Hilbert, 1900: The investigations of Cantor ... suggest a very plausible theorem: Every system of infinitely many real numbers . is either equivalent to the assemblage of natural integers, . or to the assemblage of all real numbers and therefore to the continuum . Cantor, 1885: Every perfect subset of the reals has size continuum. Prove: every uncountable subset of the reals contains a perfect subset. Theorem (Bernstein, 1908) There is X ⊂ R such that neither X nor R \ X contain a perfect subset. Hilbert’s first problem Hilbert, 1900: The investigations of Cantor ... suggest a very plausible theorem: Every system of infinitely many real numbers . is either equivalent to the assemblage of natural integers, . or to the assemblage of all real numbers and therefore to the continuum . Cantor, 1885: Every perfect subset of the reals has size continuum. Prove: every uncountable subset of the reals contains a perfect subset. Theorem (Bernstein, 1908) There is X ⊂ R such that neither X nor R \ X contain a perfect subset. The family of perfect subsets of the reals has property B: Hilbert’s first problem Hilbert, 1900: The investigations of Cantor ... suggest a very plausible theorem: Every system of infinitely many real numbers . is either equivalent to the assemblage of natural integers, . or to the assemblage of all real numbers and therefore to the continuum . Cantor, 1885: Every perfect subset of the reals has size continuum. Prove: every uncountable subset of the reals contains a perfect subset. Theorem (Bernstein, 1908) There is X ⊂ R such that neither X nor R \ X contain a perfect subset. The family of perfect subsets of the reals has property B: ∃X ⊂ R ∀ perfect P P ∩ X 6= empt and P \ X 6= ∅ Property B Bernstein: The family of perfect subsets of the reals has property B. Property B Bernstein: The family of perfect subsets of the reals has property B. Definition: Let A⊂P(X) be a family of sets. Property B Bernstein: The family of perfect subsets of the reals has property B. Definition: Let A⊂P(X) be a family of sets. A has property B iff Property B Bernstein: The family of perfect subsets of the reals has property B. Definition: Let A⊂P(X) be a family of sets. A has property B iff • ∃Y ∀A ∈ A A ∩ Y 6= ∅ and A \ Y 6= ∅ Property B Bernstein: The family of perfect subsets of the reals has property B. Definition: Let A⊂P(X) be a family of sets. A has property B iff • ∃Y ∀A ∈ A A ∩ Y 6= ∅ and A \ Y 6= ∅ • χ(A)= 2 Property B Bernstein: The family of perfect subsets of the reals has property B. Definition: Let A⊂P(X) be a family of sets. A has property B iff • ∃Y ∀A ∈ A A ∩ Y 6= ∅ and A \ Y 6= ∅ • χ(A)= 2 where the chromatic number of A: χ(A)=min{λ | there is a coloring of X with λ colors s. t. the elements of A are not monochromatic }, Property B Bernstein: The family of perfect subsets of the reals has property B. Definition: Let A⊂P(X) be a family of sets. A has property B iff • ∃Y ∀A ∈ A A ∩ Y 6= ∅ and A \ Y 6= ∅ • χ(A)= 2 where the chromatic number of A: χ(A)=min{λ | there is a coloring of X with λ colors s. t. the elements of A are not monochromatic }, χ(A)=min{λ | ∃f : X → λ ∀A ∈ A |f [A]|≥ 2}. Property B Bernstein: The family of perfect subsets of the reals has property B. Definition: Let A⊂P(X) be a family of sets. A has property B iff • ∃Y ∀A ∈ A A ∩ Y 6= ∅ and A \ Y 6= ∅ • χ(A)= 2 where the chromatic number of A: χ(A)=min{λ | there is a coloring of X with λ colors s. t. the elements of A are not monochromatic }, χ(A)=min{λ | ∃f : X → λ ∀A ∈ A |f [A]|≥ 2}. Question: Which families have property B? Almost disjoint families χ(A)=min{λ | ∃f : X → λ ∀A ∈ A |f [A]|≥ 2}. A has property B iff ∃B ∀A ∈ A A ∩ B 6= ∅ 6= A \ B iff χ(A)= 2 Almost disjoint families χ(A)=min{λ | ∃f : X → λ ∀A ∈ A |f [A]|≥ 2}. A has property B iff ∃B ∀A ∈ A A ∩ B 6= ∅ 6= A \ B iff χ(A)= 2 • A disjoint family of infinite sets has property B. Almost disjoint families χ(A)=min{λ | ∃f : X → λ ∀A ∈ A |f [A]|≥ 2}. A has property B iff ∃B ∀A ∈ A A ∩ B 6= ∅ 6= A \ B iff χ(A)= 2 • A disjoint family of infinite sets has property B. • Definition: A family A is almost disjoint iff A ∩ A′ is finite for A 6= A′ ∈ A. Almost disjoint families χ(A)=min{λ | ∃f : X → λ ∀A ∈ A |f [A]|≥ 2}. A has property B iff ∃B ∀A ∈ A A ∩ B 6= ∅ 6= A \ B iff χ(A)= 2 • A disjoint family of infinite sets has property B. • Definition: A family A is almost disjoint iff A ∩ A′ is finite for A 6= A′ ∈ A. • Does every almost disjoint family of infinite sets has property B? Almost disjoint families χ(A)=min{λ | ∃f : X → λ ∀A ∈ A |f [A]|≥ 2}. A has property B iff ∃B ∀A ∈ A A ∩ B 6= ∅ 6= A \ B iff χ(A)= 2 • A disjoint family of infinite sets has property B. • Definition: A family A is almost disjoint iff A ∩ A′ is finite for A 6= A′ ∈ A. • Does every almost disjoint family of infinite sets has property B? Theorem (E. W. Miller, 1937) ω There is an almost disjoint A⊂ ω with χ(A)= ω. Almost disjoint families with large chromatic number Theorem (E. W. Miller, 1937) ω There is an almost disjoint A⊂ ω with χ(A)= ω. Almost disjoint families with large chromatic number Theorem (E. W. Miller, 1937) ω There is an almost disjoint A⊂ ω with χ(A)= ω. Proof (the hard way): Almost disjoint families with large chromatic number Theorem (E. W. Miller, 1937) ω There is an almost disjoint A⊂ ω with χ(A)= ω. Proof (the hard way): Theorem (Balcar, Vojtáš, 1980) Every non-principal ultrafilter U on ω has an almost disjoint refinement, i.e. a family AU = {AU : U ∈U} such that ω • AU ∈ U for U ∈U • AU ∩ AV is finite for U 6= V ∈U Almost disjoint families with large chromatic number Theorem (E. W. Miller, 1937) ω There is an almost disjoint A⊂ ω with χ(A)= ω. Proof (the hard way): Theorem (Balcar, Vojtáš, 1980) Every non-principal ultrafilter U on ω has an almost disjoint refinement, i.e. a family AU = {AU : U ∈U} such that ω • AU ∈ U for U ∈U • AU ∩ AV is finite for U 6= V ∈U Clearly χ(AU )= ω: Almost disjoint families with large chromatic number Theorem (E.